have you taken a compression test of the engine ? I had a similar situation on my A-4 and found the comp. in #4 was low, the noise was, I thought,in the #3.I think your problem may be a sticky valve.Shoot a generous amount of Seafoam penetrating oil or MO-44 in the sparplug holes around the top of the valves. 24 hrs later put two or three Qts of Marvel Mystery oil in engine and run for an hr or two.Moyers Newsletter states he ran an engine with pure MM and cleaned it as slick as a whistle.Three qts for an hr cleaned my engine also.The engine, after 30 yrs now has 110-120 comp. and never ran better.